Everyone knows it’s cheaper to buy round-trip plane tickets than one-way fares. But increasingly, everyone is wrong. Travelers are purchasing more one-way flights as the price premium declines compared with round trips, according to Airlines Reporting Corp., which analyzes fare data. One-way jaunts have expanded to 42 percent of air travel so far this year, up from 29 percent three years ago, ARC said. The price drop is especially pronounced on leisure routes, with corporate travel seeing barely any change. From cities such as New York, Dallas, Denver and Los Angeles, the average premium to some destinations has shrunk to $10 from $125 when looking at each leg of the trip, ARC said. “The long-held belief that it is better to purchase round-trip tickets whenever possible to get the best fares is simply no longer true,” ARC said. The report offers no reasons for the change. One possibility is the decline of “fare fences,” such as the one-way premium and requiring a Saturday-night stay, amid rising competition from discount airlines. There’s also a potential downside. While one-way fares are increasingly popular, ARC warned of the risk of higher change fees if a customer buys two one-way tickets. “If the full itinerary is changed, the traveler may incur two change fees and that may make round-trip ticketing a better option in some cases,” ARC said in the report.